1. Field of the Invention
This invention relates to semiconductor devices and, in particular, semiconductor devices based on III-V materials.
2. Art Background
The etching of semiconductor materials in device fabrication is a well practiced art. In particular, the etching of III-V semiconductor materials has been performed by a variety of expedients. For example, both wet chemical etching (etching done utilizing a liquid etch) and dry etching (etching typically performed in a reactive gas and/or plasma discharge gas environment) have been employed.
Each of these etching techniques have their own attributes, and generally are used in the specific situations for which they are best suited. In the case of wet chemical etching, typically a relatively smooth surface on the walls of the etched feature is achieved. These smooth surfaces are desirable in applications such as those involving reflective surfaces for optical devices. (Surface smoothness for the purpose of this invention is defined with respect to the area of a series of adjoining but not overlapping specifically defined imaginary squares. These squares each 800 A on a side (or in the case of a sidewall being considered for smoothness, squares whose side is 800 A or one-half the smallest dimension of the wall, whichever is smaller) are oriented so that the sum of the distance from all points, along a chosen portion of the surface being considered for smoothness, measured perpendicularly to the square corresponding to this portion is a minimum and so that the square all together cover essentially the entire projected region of the surface being considered for smoothness, but no more. Then once the squares are defined, the sum of all the areas defined by the projections perpendicular from all the points on the surface of features overlying the square which at their extreme points are removed a perpendicular distance of no more than 500 A measured perpendicular to the square underlying the feature, plus the sum of all the areas defined by the projections perpendicular from all the points on the surface of voids underlying the square which at their extreme point are removed a perpendicular distance of no more than 500 A is divided by the sum of the areas of all the squares. (Overlying denotes the direction from the surface of the material away from the bulk of the etched material and underlying denotes the direction from the surface of the material into the bulk.) Typically, a smooth surface is one where this percentage is less than 20 percent, generally for wet etching less than 5 percent.) However, wet chemical etching such as that described in Y. Tarul et al, Journal of Electrochemical Society, 118, 118 (1971) or S. Adachi, Journal of the Electrochemical Society, 128, 1342 (1981) usually yields nonvertical etched walls, a wall with a curved surface, or an unacceptably small vertical to horizontal etched aspect ratio. (A nonvertical wall in the context of this invention is a wall which diverges from the normal to the original surface more than 10 degrees. The aspect ratio is the ratio of the maximum distance etched vertically to the maximum distance etched horizontally in any plane underneath the mask.) Therefore, wet chemical etching is generally not appropriate for applications such as mirror facets on optical devices, where vertical walls or high aspect ratios are required.
Dry etching procedures are often used precisely for the applications in which wet chemical etching is not appropriate, e.g., applications where a vertical wall with a high aspect ratio is required. Such dry etching is usually performed by striking a discharge in a gas and subjecting the material to be etched to the chemical entities formed in the discharge. The etching is ultimately achieved through chemical reaction of discharge entities with the substrate. This reactive etching is sometimes accompanied by enhanced removal of substrate material through kinetic impact of the gas discharge entities with the substrate. Although such procedures are quite advantageous in situations where vertical walls and high aspect ratios are required, generally the walls which are produced are relatively rough (typically a smoothness of worse than 30 percent). Thus, situations which require a smooth wall and lend themselves to wet chemical etching are often not congruous with dry etching. Additionally, ion assisted plasma etching often causes substantial damage near the surface of the etched material. Such surface damage can cause degradation both to optical and electrical properties while impeding subsequent epitaxial deposition. Ion etching also requires a precisely controlled smooth edged mask for processing procedures. Although such masks are producible, their preparation increases the cost of device fabrication.
For some very demanding applications, such as required in the fabrication of integrated optical components, a combination of attributes is desirable. For many devices used in optical components, a smooth and extremely flat surface is desirable, i.e., a surface in which adjoining but not overlapping squares (as defined above, but with sides of 100 A) form an angle of less than two degrees with respect to any square as defined above but with 5000 A sides. Such smooth, flat surfaces are typically not available even in wet chemical etching. Other strictures are imposed in optical component fabrication. Light beams processed in, for example, an integrated optical component typically propagate, in a direction parallel to the surface of the substrate. To avoid distortion of the propagating light with an associated loss of information, it is often necessary that the walls of optical devices in the integrated component be essentially vertical. Nevertheless, the combined requirements of essentially vertical, well-defined and extremely smooth, flat walls have not been satisfied with one process.